Quaternary ammonium hydroxides such as tetramethylammonium hydroxide (TMAH) are strong organic bases that have been known for many years. This compound has been available commercially as the solid pentahydrate and as solutions in methanol and water. TMAH has been used mainly in analytical chemistry as a titrant for acids in organic solvents and as a supporting electrolyte in polarography. These uses are based upon good solubility in alcohols and a relative freedom from alkali metal impurities. Aqueous solutions of TMAH have been used extensively as a developer for photoresists in printed circuit board and microelectronic chip fabrication. Use in the electronics area requires that there be no residue following the normal post bake period. Even traces of alkali metals would interfere in the operation of the electronic circuits. Accordingly, impurity specifications for TMAH to be used in electronics are much stricter than had been acceptable previously. Current requirements, based on contained TMAH, are: .ltoreq.4000 ppm Cl, Br, I or carbonate and .ltoreq.10 ppm each of Li, Na, K. However, it is advantageous to the electronics fabricator to employ TMAH in which the above mentioned impurities approach zero.
Quaternary ammonium hydroxides such as TMAH have been produced by various techniques. For example, aqueous solutions of tetramethylammonium halides have been reacted with a suspension of silver oxide to produce TMAH. Such procedure is considered to be somewhat laborious, expensive, and results in silver ion contamination of the final product. Another metathesis reaction utilizes tetramethylammonium chloride (TMACl) and potassium hydroxide in methanol with the precipitated potassium chloride being filtered from the methanol solution of TMAH. The resultant product contains chloride and potassium concentrations that are considered to be excessive for electronic quality applications. A third metathesis route involves the treatment of the corresponding oxalate or carbonate in aqueous solution with an excess calcium hydroxide suspension. This technique is further described in French Pat. No. 1,546,227.
Anion exchange resins have also been utilized in connection with the manufacture of TMAH from the chloride. Ion exchange techniques are considered to be acceptable for use in the production of laboratory quantities of quaternary ammonium hydroxide but are considered to be impractical for producing larger quantities due to high resin volume requirements.
A third general method involves the electrolysis of certain tetramethylammonium salt solutions. Typical of such processes include those shown in U.S. Pat. Nos. 3,402,115 and 3,523,068. These techniques involve the electrolytic treatment of quaternary ammonium salts containing non-electrolyzable anions. Other techniques included U.S. Pat. Nos. 2,363,387 and an article entitled "Preparation of Tetramethylammonium Hydroxide by Electrolysis" by S. Krishnan et al., Denki Kagaku Oyobi Kogyo Butsuri Kagaku 1971, 39 (3), 221-2 (Japan).
It is also known to utilize an oxidation-reduction cell employing a cationic membrane process for the electrolytic conversion of TMACl to TMAH. Such process is believed to represent the current state of the art for preparing low impurity level TMAH for electronics fabrication application. The transport characteristics of the cationic membrane of this known process result in an upper attainable product concentration limit of less than about 25%. Because TMAH is commonly supplied for electronics applications as a solid pentahydrate (TMAH.sup.. 5H.sub.2 O ) which contains approximately 50% TMAH by weight, TMAH solution from this known electrolytic cell process must then be concentrated from the upper attainable concentration limit of less than about 25% by use of stripping, evaporation, or crystallization processes. These additional manufacturing steps add to the complexity and cost of manufacture. Furthermore, elevated temperature concentrating processes can lead to thermal degradation of the product.
In contrast, the invention of this application discloses an improved electrolytic method for producing quaternary ammonium hydroxide such as TMAH which results in product concentrations greater than 25 wt. % and even up to 50 wt. %. The latter wt. % results in a solid TMAH product. The products of the invention are substantially free from harmful carbonates and alkali metal contaminants such as Na, Li, and K and halides such as Cl, Br, and I and are thus acceptable for use as a developer for photoresists in electronic applications.